- Multifactorial characters: many factors, such as genes and environment affect phenotype
- Pedigree Analysis: Square is male, circle is female, painted in means they have that trait
- X-linked dom → father passes to all daughters; X-linked recessive → mother passes to all sons
Relationship Among Alleles of a Single Gene 
Complete dominance: Heterozygous phenotype is the same as the
- dominant
- Incomplete dominance of either allele: heterozygous phenotype is intermediate between two homozygous
- Dominant allele doesn’t make that much protein, recessive makes none, so heterozygous makes weaker amount
- Codominance: both inherited alleles are completely expressed in heterozygotes
- People that are MM (L^M, L^M) produce the one molecule that appears on surface of blood cells, NN (L^NL^N) produce the other; and those who are MN (L^M, L^N) produce both
- Pleiotropy: one gene affects multiple phenotypic characters
- Ex: sickle cell disease
- Multiple alleles: some genes have more that two alleles
- Ex: blood group that produces A, B, and O blood types, there are 3 possible alleles → I^A, I^B, or i
- Superscripts used because the two alleles, A and B, are codominant; lowercase i is recessive when expressed with others
- There are six possible genotypes representing all possible combinations of 2 alleles: [I^A, I^A] and[ I^A, i] (A blood type), [I^B, I^B and I^B, i] (B blood type), [I^A, I^B] (AB blood type) and [ii] (O blood type)
- 4 phenotypes correspond to presence/absence of an A or B sugar component attached to plasma membrane of red blood cells (A sugar, B sugar, I^A, I^B both sugar, and ii no sugar)
Relationships Among Multiple Genes
- Epistasis: Expression of one gene affects/masks another
- Ex: hair color in labradors where B codes for melanin better (black) than b (brown). Second allele E needed to deposit melanin; ee is dysfunctional so no produced melanin is deposited (yellow)
- Polygenic Inheritance:
- Interaction of many genes that affect a single phenotype (Ex: height, very short → very tall)
- Quantitative Characters: Controlled multiple gene which vary/add up along a continuum; affected by polygenic inheritance
- g. eye color, skin color → three genes produce melanin, skin color determined by how much genes are expressed
- Any human character that is polygenic, cannot be predicted (like eye color)
Environmental Impact on Phenotype
- Environmental factors influence gene expression and can lead to phenotypic plasticity.
- Phenotypic plasticity: When the same genotype can result in multiple phenotypes under different environmental conditions
- Examples:
- At high temp & pH enzymes & transcription factors become denatured
- Environment might contain a molecule that acts as a repressor or activator
Case Study: Identical Twins
- Although have same DNA, expression of DNA influenced by environmental factors
- Before are born → experience slightly different environments in uterus → affect gene
- expression → different phenotypic expressions
Examples
- Nutrition: not enough nutrients can inhibit growth and plants without enough nitrogen may not flower
- May also influence expression of genetic disorders (lactose intolerance)
- Ex: ppl cannot metabolize specific amino acid → amino acid accumulates → brain cells die → death; minimizing amino acid → safe
- Organisms w/ mutation so cant syhtnesize amino acid can grow in environment w/ amino acid
- Temperature:
- Influences sex dertermination in some reptiles
- Eggs incubated at lower temperatures become males; those at higher temperatures become females
- Influences fur & skin color of animals, melanin production (more UV = more melanin)
- Soil pH:
- Influences flower color; blue in low pH and pink in high pH
- Released Chemicals
- Chemical signals can affect gene expression and often needed to elicit mating
- Ex: bacteria secrete signaling molecules that stimulate nearby bacteria to aggregate & form biofilms
- Ex: yeast cells only mate with yeast cells of opposite mating type; yeast releases signalling molecule (pheromone) → only opposite yeast respond
Multifactorial Diseases and Disorders
Behavior of Recessive Alleles
- Recessive allele that causes a genetic disorder (a) codes for no protein or malfunction or misfolding
- Recessively inherited: must be homozygous recessive
- Carriers: heterozygous individual with recessive allele and can pass on disease
- (Aa) usually have normal phenotype because (A) codes for enough
- Cystic Fibrosis: caused by a mutated channel gene that causes ppl with two recessive to not have chloride transport
- Pleiotropy: affects multiple organs
- Sickle Cell Anemia: mutated hemoglobin gene also recessively inherited
- Tay Sachs Disease: mutated gene codes for a defective lipid breakdown protein in the brain
- BUT at the molecular level, is incomplete dominance, bcuz only digest half the amount of enzymes
- Carriers: heterozygous individual with recessive allele and can pass on disease
Dominantly Inherited Disorders
- Lethal Dominant: only needs one copy to kill (heteroz)